situation phenomena circumstance phenomenon

Câu hỏi :

As heart disease continues to be the number-one killer in the United States, researches havebecome increasingly interested in identifying the potential risk factors (33)­­­­­­­­­­____trigger heart attacks. High-fat diets and life in the fast lane have long been known to contribute to the high incidence of heart (34)_____ . But according to new studies, the list of risk factors may be significantly longer and quite surprising. Heart failure, for example, appears to have seasonal and temporal patterns. A higher percentage of heart attacks occur in cold weather, and more people experience heart failure on Monday than on any other day of the week. (35)____ , people are more susceptible to heart attacks in the first few hours after walking. Cardiologists first observed this morning (36)____in the mid-1980, and have since discovered a number of possible causes. An early-morning rise in blood pressure, heart rate, and concentration of heart (37)____hormones, plus a reduction ofblood flow to the heart, may all contribute to the higher incidence ofheart attacks between the hours of 8:00 AM. and 10:00 AM.